Nanostructured thermoelectric energy conversion and refrigeration devices

Energy consumption in our society is increasing rapidly. A significant fraction of the energy is lost in the form of heat. In this talk we introduce thermoelectric devices that allow direct conversion of heat into electricity. A key requirement to improve the efficiency is to increase the Seebeck coefficient (S) and the electrical conductivity (σ) while reducing the electronic and lattice contributions to thermal conductivity (κe+κL). Some new physical concepts and nanostructures make it possible to modify the trade-offs between the bulk material properties through the changes in the density of states, scattering rates and interface effects on the electron and phonon transport. We will review recent experimental and theoretical results on nanostructured materials of various dimensions: superlattices, nanowires, nanodots, as well as solid-state thermionic power generation devices [1]. Most of the recent success has been in the reduction of lattice thermal conductivity while maintaining good electrical conductivity. Several theoretical and experimental results to improve the thermoelectric power factor (S2σ) and reduce Lorenz number (σ/κe) are presented. Novel metal-semiconductor nanocomposites are developed where the heat and charge transport are modified at the atomic level. Theory and experiment are compared for several III-V and nitride nanocomposites and multilayers [2]. Potential to increase the energy conversion efficiency and bring the cost down to $0.1-0.2/W will be discussed [3]. We also describe how similar principles can be used to make micro refrigerators with cooling power densities exceeding 500 watts per centimeter square [4] in order to selectively remove dynamic hot spots and decrease significantly the requirements for overall cooling of the chip. We also describe some recent advances in nanoscale thermal characterization. Thermoreflectance imaging is used to measure the transient temperature distribution in power transistors. Resolution down to 100ns in time, submicron spatial and 0.1C in temperature are achieved using megapixel CCDs. Finally, the transition between energy and entropy transport in nanoscale devices will be briefly discussed.